1. Field of the Invention
This invention relates to a method of stabilizing solutions of vanadium tetrachloride in inert organic solvents and to stabilize solutions produced by the method. More particularly, but not exclusively, the invention relates to processes for copolymerizing olefins in which catalyst is introduced into the polymerization zone in stabilized form.
2. Description of the Prior Art
Many types of catalyst have been proposed for producing amorphous co-polymers of olefins, the so called Ziegler catalysts being particularly effective. It is generally known that good results can be achieved using a Ziegler-type catalyst comprising an organoaluminum compound and a vanadium compound soluble in an inert organic solvent. However, most of the vanadium-containing catalyst systems of this type, although exhibiting a very high activity in the initial stages of the polymerization, show a rapid drop in activity after a relatively short period of time and also have a low catalyst efficiency.
Examples of vanadium compounds which have been used in polymerization processes are vanadium halides and vanadium oxyhalides, such as vanadium tetrachloride, vanadium oxytrichloride, vanadium oxytribromide and substituted vanadium halides and substituted vanadium oxyhalides, particularly those wherein the substituents are chosen from alkyloxy and aryloxy groups. Examples of such substituted vanadium halides and oxyhalides are VOCl.sub.2 (OC.sub.2 H.sub.5), VOCl.sub.2 (OC.sub.2 H.sub.5).sub.2 and tri-n-butylvanadate. Complex compounds of vanadium such as vanadium trisacetylacetonate have also been used.
In commercial polymerization processes, the most commonly used vanadium compound is the pentavalent vanadium oxytrichloride (VOCl.sub.3), but as mentioned, this and other vanadium compounds suffer from low catalyst efficiency. In order to improve this low catalyst efficiency, many catalyst activators have been proposed for use with the vanadium compound and co-catalyst system. The most effective of these activators are generally considered to be the alkyl esters of trichloroacetic and tribromoacetic acids, the ethylene glycol monoalkyl ether esters, particularly monoethyl ether esters of trichloroacetic acid or tribromoacetic acid, and esters of perchlorocrotonic acid, since these have been found to improve catalyst efficiency considerably.
To take an example of a known commercial technique, butyl perchlorocrotonate is used as a reactivator for vanadium oxytrichloride catalyst systems, which are used in the bulk polymerization of EPM and EPDM polymers, in combination with a haloaluminiumalkyl co-catalyst such as aluminium sesquihalide. It is believed that the catalyst system produced on contacting the components suffers from deactivation because of conversion of the active V.sup.5+ species to inactive V.sup.2+ species. The butyl perchlorocrotonate reactivates that V.sup.2+ to V.sup.3+ which is active as a catalyst in the system of the polymerization process.
Many publications are known which teach the use of esters of halogenated organic acids for the purpose of reactivating catalyst systems derived from the contact of vanadium oxytrichloride and an organoaluminium co-catalyst in the polymerization environment. Although some of these suggest the use of such reactivators with catalyst systems employing vanadium compounds in general, it is noticeable that in fact the specific teachings of all go to the use of vanadium oxytrichloride/organo-aluminium compound catalyst systems, because these are the only systems which can readily be used in commercial polymerization processes.
Thus, European Pat. No. 59,034 (1982; Sumitomo) is concerned with the production of ethylene-propylene co-polymers by a series reactor process, and the inventive aspects of the Sumitomo patent relate to the use of two reactors connected in series and operated at different temperature levels. The use of such reactors is said to be necessary since the catalyst activators conventionally employed in ethylene co-polymerization processes lead to the production of polymers which are deficient in parameters such as green strength, creep, cold flow and general processability. This prior art teaching discloses polymerization process conditions whereby the monomers are contacted in the presence of organo-aluminium compounds, trivalent to pentavalent vanadium compounds and, as a catalyst activator, an ester of an halogenated organic acid which may be present in a wide range of concentrations based on the vanadium. It is noted that the examples are specific on use of vanadium oxytrichloride as the vanadium component of the catalyst system (although vanadium trichloride is mentioned apparently in error at page 12 line 6). Although it is stated that the activator may be added in admixture with one or more of the catalyst components, in fact in the examples the activator is injected into the polymerization zone directly. There is no teaching to use an halogenated organic acid ester in direct combination with a vanadium tetrachloride solution.
British Pat. No. 1,444,169 (Huls) relates to a two-stage polymerization process using a titanium/aluminium catalyst in the first stage and a vanadium/alkyl aluminium sesquihalide catalyst in the second stage. To improve the activity of the catalyst a perchlorocrotonic acid ester, preferably butyl perchlorocrotonate, can be added in amounts of 1 to 2 mols per mol of vanadium oxytrichloride. Again, the prior art is specific on the use of vanadium oxytrichloride as the second stage catalyst component, and teaches injection of the activator into the polymerization zone to modify the catalyst system formed in situ by contact of the specified catalyst components.
British Pat. No. 1,403,372 (ISR) teaches the polymerization of ethylene optionally with up to 20 mol percent of another alpha-olefin, using a catalyst comprising a vanadium compound and a specific organoaluminium compound, and in the presence of hydrogen and a halogenated organic compound. The exact function of this halogenated organic compound is not clear, but it is generally used in excess with regard to the vanadium concentration. Once again, the vanadium catalyst specifically discussed and exemplified is vanadium oxytrichloride, and again this catalyst component and the halogenated compound are each injected directly into the polymerization reaction zone.
A typical liquid phase commercial polymerization process employs apparatus comprising a polymerization zone or reactor, to which are delivered the various components which constitute the reactants, solvents or catalysts. For example, in a process for producing EPDM terpolymer, there is delivered to the polymerization zone the monomers constituted by ethylene, propylene and a diene such as ENB; solvent such as hexane; and the components of the catalyst system which are typically a solution of vanadium oxytrichloride in hexane, and an organo-aluminium cocatalyst such as aluminium sesquihalide. Additional activators may also be delivered into the polymerization zone.
The polymerization is typically carried out in the liquid phase under stirred conditions, continuously, at a temperature of about 30.degree. C. The polymerization product mixture is then delivered from the polymerization zone and quenched to halt the polymerization reactions. The quenched product is then de-ashed, that is the catalyst components are removed, leaving a mixture comprising a polymer/hexane solution which also contains unreacted monomers and other process additives. Water is introduced into this system to yield a polymer/water slurry which is passed to a settler where polymer and water are separated and any diene contained therein is removed for recycling to the polymerization zone. The organic components of the polymerization products such as solvent and excess propylene are passed to a flash drum where distillation separates the hexane solvent from propylene which is recycled to the polymerization zone. Distillation generally destroys any organic additives present in the hexane, although the distilled hexane will certainly contain small proportions of monomeric such as propylene and diene. This distilled hexane is recycled to the polymerization zone, although a proportion of it is directed to the catalyst make-up facilities, where it is used to prepare the catalyst component solutions for injection into the polymerization zone.
The system described above is perfectly acceptable when using vanadium oxytrichloride as the vanadium compound catalyst component, given the acceptance of the generally low catalyst efficiency of this compound. However, it is known that vanadium tetrachloride is a much more active catalyst than vanadium oxytrichloride, but suffers the disadvantage of being more susceptible to heat and light degradation, with formation of vanadium trichloride and a chlorine radical. Vanadium trichloride is insoluble in the organic solvents typically employed in polymerization processes. Thus, a major drawback in employing the more reactive vanadium tetrachloride species as a catalyst component is that there is almost inevitably a plugging of catalyst delivery systems. For this reason vanadium tetrachloride is not widely used commercially, even though it is under certain reactor conditions three times more catalytically active than vanadium oxytrichloride. If vanadium tetrachloride could be used commercially without leading to plugging, then its improved catalyst efficiency would be a major advance in the art.
More specifically, the problems associated with use of vanadium tetrachloride are that it is a liquid which decomposes easily to solid components which clog the catalyst transfer lines and make it difficult to pump or meter accurately. The decomposition products are not useful as catalysts and the decomposition is rather difficult to stop. Indeed, it proceeds very readily on dilution of the vanadium tetrachloride with organic solvents such as hexane, and even faster in the presence of olefins such as ethylene, propylene and dienes, e.g. ENB. This is a major problem in commercial polymerization processes such as that described above, because hydrocarbons such as hexane and olefins such as ethylene and propylene are nearly always present in the recycle streams. Moreover, in systems where the organic solvent is recovered and recycled, and a proportion of it is employed to dissolve the catalyst species prior to delivery into the polymerization zone, the plant operator is effectively dissolving the vanadium catalyst species in a solvent, which itself and through contaminants contained therein, is guaranteed to promote the decomposition of the vanadium tetrachloride molecules to insoluble and useless components.
This difficulty in handling vanadium tetrachloride catalyst on a commercial basis is the reason why the prior art discussed above does not detail use of vanadium tetrachloride, and there is a long-standing desideratum in the art to provide an improved means of using this valuable catalyst component in the presence of olefins and organic solvents, especially at temperatures which are consistent with dissolving the catalyst in a recycled (distilled) solvent stream.
Investigations into the behavior of vanadium tetrachloride as a catalyst component have led to several conclusions, all of which indicate the difficulties encountered by those seeking to use the material in commercial catalysts. Thus, vanadium tetrachloride is a liquid which only undergoes a slow thermal decomposition in its pure form, the decomposition half life at room temperature being in excess of 10.sup.4 days. However, solutions of vanadium tetrachloride in organic solvents such as hexane are much less stable than the pure liquid. It is believed that the hexane readily undergoes free radical chlorination by the chlorine radical which is the primary produce of vanadium tetrachloride decomposition. This absorption of the chlorine radical tends to promote vanadium trichloride production. Indeed the decomposition of vanadium tetrachloride in hexane at 65.degree. C. is quite rapid and the rate of decomposition increase with time, being an autocatalytic reaction. The rate of decomposition of vanadium tetrachloride in hexane drops on lowering the temperature; for example, at 25.degree. C. solutions are more or less stable for up to 10 hours, and at 0 .degree.C. there is only minimal decomposition after two or three days. This behavior of vanadium tetrachloride solutions therefore mitigates against the use of the catalyst in processes wherein it is convenient to dissolve the catalyst in hexane, especially recycled hexane, without the expensive step of cooling the hexane which has been distilled to minimize contamination.
With regard to contamination, as mentioned above, recycled hexane in a polymerization process would contain a small proportion of olefins. It now has been found that in the presence of olefins the decomposition reactions of vanadium tetrachloride are accelerated. The vanadium tetrachloride is believed to coordinate to the olefin, and is then subject to a homolytic fission of the vanadium-chlorine bond to form vanadium trichloride which is insoluble in hydrocarbon solution and hence precipitates out. Stable solid decomposition products are readily formed by the addition of terminal-alpha-olefin and strained ring, internal olefins. Moreover, vanadium tetrachloride itself may lead to polymerization of the olefins in hexane solution and these polymer products may be insoluble in the case of, for example polymerized norbornene or ENB. In each case, the product of the reaction between vanadium tetrachloride and olefin moieties in hydrocarbon solution results in an insoluble product which tends to clog catalyst delivery lines and make efficient and controlled polymerization impossible. As mentioned, this is particularly the case where the organic solvent is recovered from the continuous polymerization process and recycled, since this will contain small proportions of olefin such as the monomer propylene and the monomer diene, for example ENB.